What reverse dating can tell you
A due date is an estimate that your doctor or midwife assigns, usually from the first day of your last period or from an early ultrasound. Reverse dating takes that finished estimate and counts backward. It answers a narrow question: if this is the estimated due date, what first day of pregnancy does the calendar imply, and around when was conception counted to have happened?
It does not answer when intercourse took place, which day ovulation happened or who a baby's parent is. Those need information that a single due date does not contain. The calculator is deliberately limited to the clinician's due date as input, so that the result can never contradict the dating your care team is using.
The arithmetic
Clinicians count a pregnancy as 40 weeks, or 280 days, from the first day of the last menstrual period. The nominal gestational start is therefore the estimated due date minus 280 calendar days. Conception is conventionally dated about two weeks after that start, so the approximate conception timing is the due date minus 266 days (38 weeks). This is the same 14-day difference that the standard due date rule assumes for a regular 28-day cycle. The American College of Obstetricians and Gynecologists (ACOG) describes the 280-day convention in its Committee Opinion on methods for estimating the due date.
The calculator counts whole calendar days, with no clock time and no time zone, so daylight-saving changes and your device's time zone cannot move a date by a day. Leap days are counted when they fall inside the range.
Worked example
Suppose the estimated due date is Thu 8 Oct 2026. Counting back 280 days gives a nominal gestational start of Thu 1 Jan 2026. Counting back 266 days gives approximate conception timing of Thu 15 Jan 2026. The conception date is 14 days after the start, which is exactly why a pregnancy is already two weeks old at conception when it is counted from the last period.
A second example crosses a leap year: for an estimated due date of Wed 1 Mar 2028, the start is Wed 26 May 2027 and the approximate conception timing is Wed 9 Jun 2027. If the due date is moved a week later to Thu 15 Oct 2026, both calculated dates move a week later too, to Thu 8 Jan 2026 and Thu 22 Jan 2026. Changing the due date changes both results by the same number of days, which is why the dates should always follow the due date your clinician gives.
Conception age versus gestational age
Two different clocks are in use. Gestational age starts on the first day of the last period, roughly two weeks before conception. Fetal or conception age starts at conception. When your clinician says you are 10 weeks pregnant, they mean 10 weeks of gestational age, which is about 8 weeks after conception. The reverse due date calculator shows both on one timeline so that you can see where the two-week gap comes from.
Why real dates vary
- Cycle length. The 280-day rule assumes a regular 28-day cycle with ovulation around day 14. A longer or shorter cycle moves ovulation, and so conception, earlier or later.
- Ovulation timing. Ovulation can differ from cycle to cycle even in the same person. Sperm can also survive for several days, so conception does not have to happen on the day of intercourse.
- Implantation. Conception and implantation are different events, and the calendar only dates the first.
- Redating. A clinician may move the due date after an early ultrasound. When that happens, use the new date; the reverse dates move with it.
- Assisted conception. If the pregnancy came from IVF, the transfer date and embryo age give a more precise start. Use the IVF due date calculator for that, not the reverse calculator.
Because of this, the conception date shown here is the middle of a window of days, not a single certain day. No confidence interval is given, since no single figure would honestly describe it.
More on dating a pregnancy
To go forward from a last period, use the pregnancy due date calculator. To look at fertile days in a cycle, see the ovulation calculator. More health tools are listed on the Fitness & Health calculators page. Dates you enter are never saved or sent.
Frequently asked questions
Can this tell me the exact day of conception?
No. It gives approximate timing under the conventional assumptions. Real conception can fall several days either side, and the calendar cannot see it.
Why is conception 266 days before the due date and not 280?
The 280 days run from the first day of the last period. Conception is counted about 14 days later, so 280 minus 14 is 266.
Which due date should I enter?
The one your doctor or midwife gave you. A due date calculated elsewhere can differ from the one your clinician finalised after a scan.
Does it work for IVF pregnancies?
Not as intended. For IVF, the transfer date and the embryo's age give a more precise start, and a separate calculator on this site uses them.
Does it work for twins?
The dating arithmetic is the same, but twin pregnancies are usually dated and monitored differently, so rely on your clinician's dates.
Sources and limits of this page
The 280-day convention follows ACOG, Committee Opinion No. 700: Methods for Estimating the Due Date (2017). The 14-day conception difference is the standard assumption for a regular 28-day cycle. This page is educational information, not medical advice, and it has not been reviewed by a clinician. No birth date is predicted.